1. Field of the Invention
The present invention relates to a control method and a control device of a gas turbine. More concretely, the present invention relates to a gas turbine control method and a control device thereof for controlling the gas turbine so as to prevent the gas turbine from being operated under an irregular condition due to an adjustment operation for restraining combustion vibrations during the operation of the gas turbine, which is a condition deviated from a regular operation condition based on an ideal fuel flow rate and airflow rate that are simulated at the design stage of the gas turbine.
2. Background of the Invention
For instance, in a gas turbine driving a generator (an AC generator), the airflow rate and the fuel flow rate toward the combustors of the gas turbine are determined based on the load (the power required by the generator), the ambient temperature, the ambient humidity and so on; and, the airflow rate and the fuel flow rate at-least-one point on the operation line such as a rated operation point are fine adjusted in the trial operation of the gas turbine so that the fine-adjusted airflow rate and fuel flow rate are used as the initial design data or a protocol condition. However, the period of the trial operation is limited only within a predetermined certain time span; as a matter of course, the trial operations cannot be performed for all the weather conditions that the gas turbine is supposed to encounter. In addition, the actual airflow rate and the fuel flow rate of the gas turbine after the commissioning thereof may deviate from those at the design stage or the trial operation stage due to the secular change such as the deterioration of the compressor performance or the clogging of filters.
On the other hand, the gas turbine is driven by the combustion gas produced through the continuous exothermic oxidation reaction between the fuel and the air supplied into the combustors; thereby, the combustion occasionally accompanies the combustion gas pressure fluctuations of a frequency from 10-Hz to several thousands-Hz, the combustion gas pressure fluctuations including: the combustion noise caused by the turbulent flow combustion due to the exothermic oxidation reaction, and the combustion vibration caused by the interaction between the heat dissipation due to the time lag from fuel evaporation to fuel combustion and the flame propagation speed change due to combustion gas diffusion and revolution.
When particular attention is paid to the combustion vibrations, it is recognized that the above-described interaction between the heat dissipation and the flame propagation speed change is the vibration source; and, resonance sometimes occurs between the natural frequencies regarding the interaction and the natural frequencies regarding the combustor as an air-column; thus, the combustion vibrations having own frequencies in a peculiar frequency range grow in the combustor. Such combustion vibrations as described are basically unavoidable, whether the strength of the vibration is greater or smaller; and, the strength level depends on the volume of the combustor, the geometry of the combustor, the combustion performance in relation to the combustion gas temperature, and so on.
The demand for further compact and high-powered gas turbines becomes remarkable nowadays. Accordingly, the combustion temperature becomes higher and higher. In order to cope with increased thermal stresses as a result of the increased temperature due to the load fluctuations of the gas turbine as well as the rapid temperature increase in the combustor, the heat resisting steel of high strength is used for the combustor and the members therearound; at the same time, in order to reduce the time and manpower regarding delivery, installation, inspection and so on, the configuration members of smaller thickness and lower rigidity are used, even though the strength itself of the member material is increased. As a result, in a case where unexpected excessive combustion vibrations occur, or in a case where the resonances between the combustion vibrations and the air-column vibrations in the combustor happen, a possibility arises that cracks may be produced on the combustor; the support member may be seriously damaged; the life expectancy as to the configuration members of the gas turbine or the combustor may be reduced.
Since such combustion vibrations as described hinder the operation of the gas turbine to a great extent, the countermeasures to restrain or evade the combustion vibrations are strongly required as far as possible, in view of plant protection and availability enhancement. Thus, it is essential that the skilled engineers take care of the control system of the gas turbine and confirm the operation stability several times a year so that the combustion stability is maintained and the combustion vibrations do not occur. This practice, however, increases maintenance cost as well as decreases operation availability.
Against the above-described problems, for instance, the patent reference 1 (JP1997-269107) discloses a combustor combustion vibration control device and a method thereof for restraining the combustion vibrations caused by the pressure fluctuations in the combustor, whereby the device and the method comprising: a frequency analyzing means or process in the case of the method invention that performs frequency analyses as to the pressure fluctuations of the combustion gas, a central processing means or process in the case of the method invention that computes the conditions to stabilize the combustion vibrations based on the frequency band in which the result of the pressure fluctuation frequency analyses exists, the pressure fluctuation frequency analyses being performed by the frequency analyzing means; a voltage amplification means or process in the case of the method invention that amplifies the outputted signals outputted by the central processing unit; a controlling means (or process in the case of the method invention) that transforms the amplified signals amplified by the voltage amplification means into the order signals to open/close the fuel valves, and transmits the order signals toward the fuel valves so as to control the fuel valve.
The combustion vibration control device and the method disclosed by the patent reference 1 treat with or are focusing on the combustion vibrations of a lower frequency. On the other hand, the frequencies of the combustion vibrations occurring in a gas turbine cover a range from a low frequency around 10-Hz to a high frequency of several thousands-Hz; moreover, a plurality of kinds of the combustion vibrations often occurs in a plurality of frequency bands at the same time. Accordingly, if the air fuel ratio as to the gas turbine operation is controlled based on the vibration countermeasure focusing on the low frequency band as per the approach disclosed by the patent reference 1 (JP1997-269107), then there is a possibility that the combustion vibration status in other frequency bands may become worse.
Against the background of the above-described problem, the applicants of this specification proposed a gas turbine control device for effectively restraining the combustion vibrations of the gas turbine, in the patent reference 2 (JP2005-155590); in the proposed approach, an order of priority is predetermined, the order of priority being related to which frequency band out of a plurality of frequency bands should be treated so as to restrain the combustion vibrations; in response to the predetermined priorities (priority orders), the gas turbine operation is adjusted so that the combustion vibrations in a higher priority frequency band is controlled; if there arises an operation condition change as to the gas turbine after the control adjustments, the database reflects the operation condition change; and, if similar combustion vibrations are experienced during the gas turbine operation, the data stored in the database can be effectively used so as to restrain the combustion vibrations. Further, in a case where sufficient data are not accumulated in the database just after the gas turbine plant is commissioned, the information data stored in a basic data section or a knowledge data section is used so as to restrain the combustion vibrations, the basic data section storing the information data as to the operation countermeasure data obtained from the operation of other gas turbines of the same type, the knowledge data section storing the information data as to the operation countermeasure data obtained from the experience of the skilled operators who are in charge of the gas turbine operation adjustments. Moreover, in a case where sufficient data to adjust the gas turbine operation are not accumulated because of the continuing stable operation condition for a long time span, the optimal operation conditions are automatically searched by periodically changing the actual operation conditions so as to accumulate the information data for the database. In this way, the gas turbine control device or the corresponding method for restraining the combustion vibrations is proposed in the patent reference 2 (JP2005-155590) so that the combustion vibrations are effectively restrained (controlled) even in a case where the combustion vibrations occur in a plurality of frequency bands.
Summing up the main points as to the patent reference 1, the control device and the method thereof treat with the combustion vibrations of a lower frequency; therefore, if the air fuel ratio as to the gas turbine operation is controlled based on the vibration countermeasure focusing only on the low frequency band, then there is a possibility that the combustion vibration status in other frequency bands may become worse.
On the other hand, according to the disclosure of the patent reference 2, the combustion vibrations relating to the high priority frequency bands can be effectively restrained; however, there may be still a problem; namely, unstable combustion phenomena may happen, for example, due to the aged deterioration of the gas turbine.
In order to evade this problem, according to the patent reference 2, the optimal operation conditions are automatically searched by periodically changing the actual operation conditions on the basis of the mathematical models that are represented by the accumulated (information) data in the control device or the database; further, new information data are retrieved through the automatic optimal operation condition searching so that the control adjustments are performed and the optimal operation condition is surely achieved. However, it is currently known that the automatic change of the actual operation conditions may become a potential cause of unpredictable load fluctuations.
The explanation is now given about the unpredictable load fluctuations as described above, by use of FIGS. 14, 15, 16 and 17. In FIG. 14, the lateral axis and the vertical axis denote the load and the pilot ratio, respectively; the pilot ratio hereby means the ratio of the pilot fuel flow rate to the total fuel flow rate as described later. As shown in FIG. 14, the pilot fuel ratio or its transition is usually predetermined with regard to the required load. Further, in FIG. 15, the lateral axis and the vertical axis denote the elapsed time and the pilot ratio, respectively. As shown in FIG. 15, the pilot ratio is automatically changed so that the pilot ratio fluctuates toward positive (+) adjustment direction as well as negative (−) adjustment direction; thereby, the information data as to the latest combustion vibration conditions are accumulated in the database; on the basis of the accumulated information data, the control is performed. Incidentally, hereafter, the term “automatic searching” will be used so as to mean obtaining the information data by changing an operation parameter such as the pilot ratio.
FIG. 16 shows a result of an operation condition, the result being brought by the adjustment of the pilot ratio; in FIG. 16, the lateral axis and the vertical axis respectively denote the load [unit %] and the pilot ratio, as is the case with FIG. 14. In FIG. 16, the line graph with the symbols ♦ is the same line graph as in FIG. 14, while the line graph with the symbols ▪ shows the transition of the pilot ratio that is brought the automatic searching according to FIG. 15. Thus, the automatic searching, the automatic pilot ratio change as a change of a control input, results in a change of the line graph as to the pilot ratio transition as shown in FIG. 16. However, the automatic searching also brings unpredictable load fluctuations around the load to be kept constant, as shown in FIG. 17 where the lateral axis and the vertical axis denote the elapsed time and the load [unit %]. In this way, a gas turbine operation may be caused, the actual operation point being deviated from the ideal operation point at which the operation state variables such as the ideal fuel flow rate and the airflow rate are to be kept as the operation state variables determined at the initial design stage.
In view of the problems as described above, the present invention aims at providing a gas turbine control method and a device thereof, an optimal operation condition for the gas turbine being automatically searched, whereby the gas turbine is able to be prevented from continuing the operation status which is deviated from the ideal operating points regarding the actuating variables such as the ideal fuel flow rate or the airflow rate that are assumed in the design stage of the gas turbine, and the design performance and the design operation status that are assumed at the design stage in relation to the ideal fuel flow rate or the airflow rate as to the gas turbine are maintained.